Molding compositions are useful in the electronics industry to encapsulate electrical components so as to protect them from electrical and environmental damage. However if the thermal conductivity of the composition is too low the encapsulant may itself be detrimental in that it may act as a heat barrier and cause the temperature of the electrical components to rise above the temperature reliability specifications for the components. This will shorten the useful life of the encapsulated electrical components particularly for components such as semiconductors. The heat dissipation problem in microelectronics is becoming increasingly important as the demands for denser and faster circuits intensify. Polymer compounds with high thermal conductivity are also useful for other products such as computer cases, battery cases, electronic controller housings and for other encasements where heat removal is an important consideration.
Conventional molding compositions include epoxy based polymers filled with fused or crystalline silica. Silica is the predominant filler presently used in thermal molding compounds due to its low cost, low thermal expansion, and low electrical conductivity. However, both types of silica are poor thermal conductors independent of the polymer in which the silica is filled. Other filler materials have also been investigated including ceramic fillers such as aluminum oxide, aluminum nitride and boron nitride. To date the maximum thermal conductivity for commercially available materials remains substantially below about 5 W/mK independent of filler material and/or epoxy resin formulation. In fact most commercially available molding compounds presently used in plastic microelectronic packaging typically have thermal conductivity values around 0.7 W/mK. Higher thermal conductivities have been reported in the literature such as for example in the 1988 IEEE by Bujard entitled Thermal Conductivity of Boron Nitride filled Epoxy Resins: Temperature Dependence and Influence of Sample Preparation in which a formulation of alumina loaded bisphenol-F epoxy resin is reported having a thermal conductivity of up to 4.5 W/mK.
Boron nitride is a known substitute for fused silica (SiO.sub.2) to provide low thermal expansion and high electrical resistivity. Boron nitride as well as aluminum nitride and aluminum oxide should theoretically provide a thermal conductivity higher than fused silica at high loading concentrations. However, to date the highest loading attainable for a boron nitride filler in an epoxy resin has been between about 65 to 75 wt. % and then only when additives and/or modifiers are included to improve the flow characteristics of the compound. Additives and modifiers increase cost and may affect other properties such as strength. In this regard boron nitride epoxy formulations have been reported with thermal conductivities in the range of 5 W/mK and up to 13 W/mK using additives to increase the ease of processing.